462 On a Contrivance to help defective Vision. 



into one. On the principle above explained, of the admiasioUjr 

 through the small aperture, of a portion of direct rays only, tlie 

 peculiar quality of a defect of vision ceases to he of any import- 

 iince, the contrivance being equally adapted to short-sighted 

 persons having a great convexity of the eye, or to those whosi' 

 natural lens has been flattened by time. The cases which have 

 already presented themselve are in full proof of the etlicacy oi' 

 the metallic spectacles. In three instances of mal-conformatiou 

 Avhere concave glasses have not afforded any relief, they have 

 enabled those on whom the trials were made to distinguish ob- 

 jects clearly, thus exercising an organ of the use of which they 

 had been hitherto deprived. 



Further trials will, I have no doubt, give a wider scope to theif 

 application. As they are calculated to meet any state of defec- 

 tive vision, where the two eyes, instead of being alike, are, as 

 I'requently occurs, of an entirely different conformation, they 

 may, with their aid, be brought into equal exercise. In strabis- 

 mus or squint, where the distortion of the pupil, as generally 

 happens, lies chiefly, if not wholly, on one side, the small aper- 

 ture may on that side be gradually brought in an oblique line, 

 towards the centre, until the pupil by habit finds its true station, 

 by its efforts to gain the light. There will be a less strain on 

 the optic nerve, which is always affected in tiie case of thes^e 

 distortions; and it will recover its tone in proportion as the 

 pupil takes its right position. It is needless to observe that the 

 exercise of vision should be confined to the side on which the 

 distortion lies, the other eye being covered by an imperforated 

 metallic plate, so as to shut out its view. Where the distortion 

 is on both sides, the remedy is obvious. 



As a substitute for an eye-glass, a single metallic spectacle 

 will be found of great utility, wherever the light is strong enouglt 

 to admit of its use. Without-doors it can be constantly em- 

 ployed with the best effect ; and has the advantage of durability, 

 not being liable to any accident. It frequently happens that an 

 eye-glass adapted to a particular defect of sight cannot be 

 found. 



The smallness of the field which presents itself in looking 

 through the metallic spectacles is an inconvenience which will be 

 remedied by use. They can only be employed where there is a 

 sufficient light ; but this light being chastened in a considerable 

 degree, they become preservers wherever a powerful glare is to 

 be encountered. Where the sight has been much impaired by 

 time, it may be recommended not to exercise it in a broad light, 

 unless through this chastened medium. The strain on the optic 

 nerve will be thus avoided. In southern climes, where there h 



